Scratch recorder



Nov. 23, 1937.

A. V. DE FOREST SCRATCH RECORDER Filed Dec. 4, 1934 Patented Nov. 23,1937 2,099,725

UNITED STATES PATENT OFFICE SCRATCH BEOOBDEB Alfred V. dc Forest,Soutliport, Com, a-ignor to Baldwln-Soutliwark Corporation; acorporation of Delaware Application December 4, 1984, Serial No. 755,913

6 Claims. (Cl. 284-70) This invention relates generally to an improvedFig. is an enlarged end of the marker with its apparatus for determiningand recording strains abrasive points. to which either a. specimen or alife size member In the illustrated embodiment of the invention may besubjected and relates more particularly which is shown herein merely forthe purpose of to an improved simplified scratch recorder. disclosingone specific form among possible others 5 It is one object of myinvention to provide an that the invention might take in practice, Ihave improved scratch recorder that is extremely simprovided a recordreceiving member I and a pie, compact and inexpensive whereby it may bemarker member 2. The member I is hereinafter readily applied to large orsmall members or to referred to as a target and the invention is re- 0moving members, one example being revolving ferred to as a scratchrecorder, for the reason airplane propellers. A further object is topmthat these terms have come into usage with the vide an improvedsimplified recorder that is gumnew field of applicant's invention. Bothof these ciently inexpensive, without sacrifice of its preelements arepreferably made of thin sheet metal cision qualities, that it may beused for a relaadapted to-be stamped with dies. The target I tivelysmall number of tests and then discarded has two elongated arcuateopenings 3 and 4 and if desired. A further object is to provide such aan intermediate clip bar 5 while a polished record scratch type recorderadapted to be made from surface 6 is provid d preferably on the Outerend relatively thin sheet metal, and in this respect of the target I.The target is normallyheld in the recorder comprises a target andmarker, the position on a specimen or other member to be tar et memberbeing constructed so that an intested, by sc ews pass t ou lon itud na20 I tegral part thereof, specifically a clip bar, coand lateral slots 1and B.

operates directly with a combined marker and The marker 2 consists of anarm 9 extending biasing spring. As a result of the simplicity oi.beneath clip bar 5 an hen p f rab curving these two elements and theirextremely light upwardly at l0 and downwardly at its elite! weight andlow cost, it is possible to applythe marking end Ii. The inner end ofarm 9 is con- 25 same directly to life size members such as pronected bya comb ed spring a fulormn I! with .peller blades having a relativelyhigh rotative a holdin e d I which s normally fixed to the speed orother moving members as well as t specimen or other member to be tested,as by a stationary members or specimens. screw passing t ou Op It Willbe so I employ the principle that friction of rest is noted that the ed81 8 d u u I2 greater than moving friction and then use a biasis merelyan upturned Portion the Sheet metal ing force on the marker less thanthe friction of 1011111118 t e e e e and in Order -P rest but greaterthan the moving friction thereby po t to funct o as a fulcrum. 8- netehpermitting relative transverse feeding movement is cut in the flatPortion the metal- 36 between the marker and target upon occurrence Themarking end II is provided with grains of relative longitudinal movementbetween the hi my suitable abrasive material that Wm same caused bydeformation of a specimen under scratch iihe record Surface that Scribe8 stresses of various kinds. Enlargement of the record theremli suchbeing highly scratch record by a Horoscope permits accurate ished andsuitably protected if desired by a 40 determination of the magnitude ofthe deformaeummedpap" 8MP ready use when such tions'and also thevariations in sequence with paper is removed Preferably the abrasive ism which the strain occurs. Various other characthe form of grams ofcammndum which may be teristics which are desirable in determiningchar-- from one to several in number spabed on the acteristics 'ofmaterial may also be obtained: .under surface of end These grams whetherof carborundum, diamond chips or other suitable Other objects andadvantages will be more ap- 1 parent to those skilled in the art fromthe i'olgrai :igg ng i zififfis a smtab 6 matrix lowing description ofthe accompanying drawing In operation the elements I and 2 are securedwhich: to a member to be tested in their relative posi- Fig. l is anenlarged view of the instrument; on as shown in p 1 ew being i t d 50 Fi2 i a fra m n ary p p tiv of a through opening l4 and slots 1 and a. Thep blade Showing the application of y scratch arm 9 is then manuallymoved -in a proved instrument; clockwise direction to one end 01' slots3 and 4. Fi 3 nd 4 are enlargements oi the scratch Theharm 9, in thusmoving, fulcrums around 56 records obtained; portion [2. and at the sametime this portion is 66 subjected to a spring tension due to the holdingend I 3 being secured to the member. The scratch arm 9 is held at theclockwise end of slots 3 and 4 due to frictional contact between arm 9and clip bar 5 and between the scratch points and the record surface 6.This is static friction or friction of rest which creates a sufficientforce to prevent spring l2 from moving arm 9 in a counterclockwisedirection. However, when the specimen or member is elongating under atension stress, or is contracting under a compression stress, thenrelative longitudinal movement occurs between the marker and target.During such movement the friction above-mentioned is reduced from staticfriction to moving friction (dynamic friction) and this moving ordynamic friction is not sufficient to resist the lateral force of springl2. As a result, said spring laterally moves arm 9 in a counterclockwisedirection about portion l2 as a fulcrum. The extent of counterclockwisemovement of course; very small as the moving friction at least retardsrapid movement, and when there is no further relative longitudinalmovement between the marker and target, the moving friction isinstantaneously transformed into static friction which immediatelyprevents further lateral movement of the marker by spring [2. At thismoment a reversal of stress from tension to compression or fromcompression to tension, or a variation in the stress, usually occurs inthe specimen or member under test, and hence the fibers of the specimenundergo further relative longitudinal movement in'a direction oppositeto that which originally occurred. Hence this relative movement againtransformsthe static friction into dynamic friction with the result thatspring l2 can again laterally move the marker arm 9 in acounterclockwise direction. Again this lateral movement is of smallmagnitude. As a result of such variations in stress, the marker arm 9 isgradually successively moved in a counterclockwise direction until therecord is complete. Thereupon the target I is removed from the member,and the marks I6 thereon are enlarged as indicated in the illustrativerecords of Figs. 3 and 4. The records under various conditions and testsmay vary widely as is also illustrated in Figs. 3 and 4.

Scratch records such as above described may be obtained in a greatnumber of applications especially on moving members such as an airplanepropeller blade diagrammaticaly indicated at 19 in Fig. 2. Here a seriesof instruments 29 is placed in longitudinal alignment from hub to tip ofthe blade while a few instruments 2| may be placed in parallel relationbut transversely of the blade.

While only one scratch record is shown in each of Figs. 3 and 4, yet itwill be understood that preferably a number of individual grit particlesare embedded in the matrix of rubber on the under side of the marker endH. The number and position of these scratching points are somewhathaphazard but in the great majority of cases three types of scratchesare recorded. Some are too deep and ragged for good measurement, someare suitable for use and some are too thin to be readily seen, much lessphotographed. Occasionally a record will be obtained where all thescratches are too deep or where the good ones are obscured by the badones. For this reason it is sometimes desirable, and also to serve as acheck,-

to use two recorders on each location of measurement wherever possible.It further occasionally happens that a grain which cuts a good record ofrepeated strains does not cut a good zero line such as 22 when the armis manually moved out from the center of clip 5 at the beginning of arun. In this case, the relation between zero and the strain can often beobtained from a difierent scratch provided that there are two or morescratches available. Hence if one scratch is not sufliciently clear soas to be suitable for a zero line, then the probabilities are that asecond scratch will be clear. Thus it is seen that one grit particle mayfunction to make a zero line and another grit particle may function torecord the strain. The best records are thus where a number of markingpoints are distributed widely. The thin tip I! of the scratch armcarrying the abrasive must therefore lie as flat as possible on thetargetand this may be accomplished by a little adjustment depending onconditions of mounting. The transverse movement of the scratch arm isregulated by the stiffness of. the spring and the friction as previouslymentioned: The friction can be widely varied by bending the bar 9 to ahigher or lower level as compared with the level of the target. Ifseveral thousand stress variations are to be recorded and the motion isa simple one in which harmonics are of no interest, the friction can behigh and the transverse travel small as is represented in the closelypacked record shown in Fig. 3. If on the other extreme hand, the problemis one of harmonic analysis, the scale must be opened up to show thescale of small deformations such as shown in Fig. 4 which illustratesthe interference portion of a record from an engine driven propellervibrating simultaneously in several modes. In

general, high rates of vibration particularly in the presence of impactrequire high friction and high spring pressure. Less drastic conditionscan be met with lower friction and longer records obtained by weakeningthe spring portion l2 as by reducing its thickness with a small file.This reduction should preferably be local to concentrate the bend asnearly at a point as is practical.

Various conditions of operation will dictate to one skilled in the useof my improved recorder the manner in which the instrument may be mosteiiiciently and effectively used. Also by proper analysis of thecharacter of scratch, it is possible to determine if shear or torsion ispresent as well as other types of stresses. It is thus seen that I haveprovided an extremely simple and yet most highly effective precisioninstrument capable of accomplishing results especially in connectionwith live moving members that have been heretofore substantiallyimpossible of accomplishment.

It will of course be understood that various changes in details ofconstruction and arrangement of parts may be made by those skilled inthe art without departing from the spirit of the invention as set forthin the appended claims.

I claim:

l. A scratch recorder comprising a target having a clip bar and arecording surface, a marker arm extending beneath said clip bar and ontop of said recording surface, means whereby said marker arm and targetmay be attached to a specimen subjected to strain variations thereby toeffect relative longitudinal movement between said arm and target, andmeans for biasing'said marker arm so that it traverses said surface uponoccurrence of said relative movement between said marker arm and target.

2. A scratch recorder comprising a sheet metal target and marker arm.means whereby said sheet metal elements have provision forfmaintaining apredetermined degree of static and dynamic frictional contact betweenthe same, whereby said marker arm and target may be attached to aspecimen subjected to strain variations thereby to elect relativelongitudinal movement between said arm and target, and biasingmeans foreiiecting relative feeding movement between the arm and target.-

3. A scratch recorder comprising a sheet metal target having a recordingsurface and two transverse slots with a. clip bartherebetween, a markerarm extending through said slots' and beneath said clip bar and restingon said recording-surface, and means whereby said marker arm and,

endsofsaidarmandwhoaethicknesscomtimtheentirewidthofthearmatthatpointwherebysaidportionmayiunctionasaf'ulcrmnand pring.

5. As an article of manufacture. a target for ascratch recorderoompridngasheetmetal memberhavingarecordingsurfaceandtwotranlvcneslotswith a clip bar formed therebetween.

6. A scratch recorder comprising a record receiving member, a markerhaving a spring portion formed as a part thereof to effect relativevfeeding movement between the marker and reoomd receiving member, meansfor attaching said record receiving member and marker to a speci'-' menso as to eil'ect relative longitudinal movement between the marker andrecord receiving munher during stress variations in the'specimen, saidrecorder elements being arranged to have static friction when nolongitudinal movement occurs and to have dynamic friction uponoccurrence of said longitudinal movement, the dynamic friction allowingsaid spring portion to eifect said feeding movement and the staticfriction preventing such feeding movement automatically in accordancewith stress variations in the specimen.

ans-ammonium.

